Impedance Changes and Fibrous Tissue Growth after Cochlear Implantation Are Correlated and Can Be Reduced Using a Dexamethasone Eluting Electrode

Abstract

BackgroundThe efficiency of cochlear implants (CIs) is affected by postoperative connective tissue growth around the electrode array. This tissue formation is thought to be the cause behind post-operative increases in impedance. Dexamethasone (DEX) eluting CIs may reduce fibrous tissue growth around the electrode array subsequently moderating elevations in impedance of the electrode contacts.MethodsFor this study, DEX was incorporated into the silicone of the CI electrode arrays at 1% and 10% (w/w) concentration. Electrodes prepared by the same process but without dexamethasone served as controls. All electrodes were implanted into guinea pig cochleae though the round window membrane approach. Potential additive or synergistic effects of electrical stimulation (60 minutes) were investigated by measuring impedances before and after stimulation (days 0, 7, 28, 56 and 91). Acoustically evoked auditory brainstem responses were recorded before and after CI insertion as well as on experimental days 7, 28, 56, and 91. Additionally, histology performed on epoxy embedded samples enabled measurement of the area of scala tympani occupied with fibrous tissue.ResultsIn all experimental groups, the highest levels of fibrous tissue were detected in the basal region of the cochlea in vicinity to the round window niche. Both DEX concentrations, 10% and 1% (w/w), significantly reduced fibrosis around the electrode array of the CI. Following 3 months of implantation impedance levels in both DEX-eluting groups were significantly lower compared to the control group, the 10% group producing a greater effect. The same effects were observed before and after electrical stimulation.ConclusionTo our knowledge, this is the first study to demonstrate a correlation between the extent of new tissue growth around the electrode and impedance changes after cochlear implantation. We conclude that DEX-eluting CIs are a means to reduce this tissue reaction and improve the functional benefits of the implant by attenuating electrode impedance.

Comparable Electrode Impedance and Speech Perception at 12 Months after Cochlear Implantation Using Round Window versus Cochleostomy: An Analysis of 40 Patients

“…Thus, the results of the study might not indicate the impact of the RW and cochleostomy approaches on electrode impedance. Some studies showed that increased impedance had a correlation with fibrous tissue growth around the electrode [28,29]. In this study, there was no significant difference in the impedance between the RW and cochleostomy groups using the same electrode type at 4 time points, which may indicate that different surgical techniques might have similar degrees of fibrous growth.…”

“…The minimal access CI group used dexamethasone before they inserted the electrode. However, some studies had shown that dexamethasone can significantly reduce tissue fibrosis, which would reduce electrode impedance [28]. Thus, the results of the study might not indicate the impact of the RW and cochleostomy approaches on electrode impedance.…”

Delayed changes in auditory status in cochlear implant users with preserved acoustic hearing

“…Fibrous tissue growth, which was an underlying etiology we suspected might influence electrode impedance, is often more extensive in the base of the cochlea compared to more apical regions (e.g. Wilk et al 2016). To the extent that measured electrode impedance reflects the extent of tissue growth, impedance measures of electrodes located in the base might be more informative than more apically located electrodes.…”

“…Electrode impedance provides information about the status of the electrode and the surrounding environment, and has been shown to be sensitive to changes in tissue growth around the electrode array (Wilk et al 2016). Histological analysis of a temporal bone from a deceased CI recipient who experienced a complete loss of acoustic hearing in the implanted ear between one and four months after implantation revealed that although post mortem hair cell and neural counts were not significantly different for implanted and unimplanted ears, the implanted ear presented with extensive fibrous tissue in the cochlear scalae.…”

Electrically Evoked Compound Action Potential Recording in Peripheral Nerves

“…There are changes which occur over time and this further complicates the relationship. Significant impedance changes occur postoperatively for implanted electrodes [49] and this has motivated the development of steroid-eluting leads to prevent this [50].…”